Thesis
Preparation and characterisation of poly(ethylene terephthalate) nanocomposites
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2011
- Thesis identifier
- T12874
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Polymer nanocomposites are considered the next generation in polymers due to the belief that the incorporation of a nanoclay into a polymer matrix may enhance properties such as gas permeability, resistance to degradation and chemical attack or elastic modulus. Numerous complications however during the research and development stages have resulted in little breakthrough and production of a polyester nanocomposite on a commercial scale in particular is considered somewhat of a ‘holy grail’. Issues vary considerably at each stage of production and include: ensuring compatibility between the nanoclay and the monomer or polymer, degradation of surface modifiers during synthesis and processing, incorporating the nanoclay at a desirable loadings, brittleness of polymer film on melt processing and clarity in the final polymer nanocomposite film. As a result of these issues there has been a rapid rise in the number of polymer nanocomposite research groups in the past few decades, a larger number of which are on this last crusade to find the holy grail in the field of polymer science. An investigation into nanoclay and monomer compatibility was the first study performed within this thesis as compatibility was crucial in ensuring nanoclay exfoliation and intercalation. If exfoliation and intercalation were not successful in the liquid monomer there would be little point in continuing the in situ polymerisation procedure method with the aim of preparing a polymer with nanoclay dispersed throughout the matrix. The study focused on a range of commercially available, surface modified nanoclays marketed under the trade names Cloisite® and Garamite®. All surface modifiers were based upon an alkyl ammonium; however a range of surface hydrophobicities were created through varying the structure of the alkyl chains and the modifier surface concentration. The dispersing agents were selected in order to examine the influence of permittivity and polarity, both of which were observed to influence exfoliation to a substantial degree. Permittivity was as a variable to explore as it influences the ability of a dispersant to shield or dampen the attractive interplatelet forces which would result in the reformation of tactoids. A high permittivity related to an effective shielding ability. Polarity was observed to give an indication of the extent of interactions possible with the nanoclay surface and/or surface modifiers through interactions such as hydrogen bonding. Subsequently, the in situ polymerisation method was investigated in order to prepare a series of poly(ethylene terephthalate) based nanocomposites. Foaming due to the degradation of the organic modifiers was observed as expected during small scale laboratory tests, and as a result an antifoaming agent was incorporated in order to manage the reaction more effectively. After a series of trials at Strathclyde, poly(ethylene terepthalate)-Garamite® nanocomposites were successfully prepared at nanoclay loadings of 0.5% w/w and 1.0% w/w at DuPont Teijin Films’ research and development facility at Wilton. The incorporation of nanoclay at loadings higher than 1.0% w/w was unviable as the foaming became uncontrollable. In addition, the polymer film exhibited an increase in brittleness during melt processing to uniaxial film, and as such a nanoclay loading higher than 1.0% w/w would again have been unfeasible. Characterisation of the polymer nanocomposites by 13C NMR, 1H NMR and FTIR confirmed the Garamite® had no effect on the polymers structure during synthesis. Determination of the molar mass however through both GPC and intrinsic viscosity measurements illustrated the degradative effect of the Garamite® during melt processing within an oxidative environment. In addition the nanoclay content dispersed within the polymer was not observed to be directly proportional to the extent of degradation and decrease in molar mass. Characterisation of the Garamite® nanoclay during a three month exchange at the University of Ottawa allowed the nanoclay group and identity to be determined. Previously, Garamite® had been suspected to consist of both a plate-like and a fibrous aluminosilicate nanoclay. The predominant nanoclay however was identified as sepiolite, a fibrous aluminosilicate belonging to the hormite group. A study into the crystallisation behaviour of the polymer and polymer nanocomposites illustrated the Garamite® acted as a heterogeneous nucleating agent. With respect to the neat polymer, crystallisation of the amorphous chip was hampered due to a high molar mass which caused a decrease in chain mobility. The nucleating ability of the Garamite® however was observed to offset the molar mass effects and act as the dominant influence over the crystallisation process. Experimentally, the nucleating ability of the Garamite® was observed through the shift in the crystallisation peak on cooling. This illustrated the polymer crystallites were able to melt and re-crystallise before the maximum temperature at which a crystallite could exist in the melt was reached. In order to investigate the mode of crystallite growth the crystallisation kinetics were determined for both the amorphous chip and uniaxial film. The modified Avrami model in particular illustrated the differing crystalline growth morphologies for the pure polymer and polymer nanocomposites. It was illustrated the pure polymer increased in dimensions from a disc-like to spherulitic crystalline morphology. The polymer nanocomposites however both exhibited sheaf like growth, illustrating heterogeneous nucleating ability of the sheaf-like fibrous sepiolite nanoclay. In contrast the Ozawa model illustrated that the mode of crystalline growth for the pure polymer did not alter as a result of melt processing and suggested a more sheaflike growth morphology. The polymer nanocomposites were also suggested to decrease in crystalline dimensions from spherulitic to disc-like lamellae. It was concluded that the modified Avrami model best described the crystalline growth morphologies of the polymer and polymer nanocomposites under study due to knowledge of the sepiolite nanoclay structure. With respect to the thermal degradation behaviour, the presence of the Garamite® was observed to have no effect on the thermogravimetric and energetic degradation of the bulk polymer matrix. Only the onset of energetic degradation was effected and was observed to decrease. It was suggested that the onset of degradation indicated the thermal stability at the polymer and nanoclay interface, and that the Garamite® was accelerating degradation in some manner. In addition, an investigation into the thermal degradation kinetics also suggested that the Garamite® accelerated degradation. An examination of the Arrhenius parameters also suggested that the effect of the Garamite® on the kinetics was not proportional to Garamite® loading, and that there appeared to be a limit to the effect of the Garamite® on degradation. An investigation into the thermo-oxidative degradation behaviour illustrated that the systems which possessed the highest maximum temperatures of degradation also possessed the highest molar masses. As a result the samples possessed a high melt viscosity which slowed oxygen diffusion and delayed the degradation process. Samples which contained the Garamite® nanoclay were also observed to possess a higher maximum temperature of degradation due to a further impedance of oxygen diffusion. It was noted at the highest Garamite® loading that the barrier effect was counteracted within the uniaxial film samples, which suggested the Garamite® accelerated the thermooxidative process. Stabilisation was also observed however through delays in the energetic onset of degradation. No effect on the degradation of the bulk polymer matrix was observed however, indicating that the degradation of the bulk polymer was neither inhibited nor catalysed, and that only the physical transport of the oxygen and small organics was affected. These competing effects were observed in the degradation kinetics. The Arrhenius model and ASTM 1641 illustrated the catalytic effect of the Garamite®, whereas the physical barrier effect was observed through the Kissinger model and ASTM E698. The final study provided insight into the thermal degradation products and mechanisms though thermal volatilisation analysis. The primary degradation products were identified as carbon monoxide, carbon dioxide, acetaldehyde, water and benzaldehyde. It was observed during degradation of the polymer nanocomposites that the evolution of the minor products ethene and acetylene was inhibited. This was attributed to some degree to the barrier effect by the nanoclay which slowed the transport of the volatiles through the polymer matrix. Mass spectrometry also revealed that the presence of the Garamite® drives the production of acetaldehyde during degradation, and therefore alters the degradation mechanism of poly(ethylene terephthalate). It was suspected that the polymer nanocomposite undergoes a heterolytic degradation mechanism as an increase in acetaldehyde was observed without an accompanying increase in carbon dioxide. Finally, the catalytic activity of the Garamite® was attributed to the Brønsted acid sites on the nanoclay which were predominantly produced during the degradation of the ammonium surface modifiers through the Hoffman elimination. The presence of Brønsted acid sites was significant as they allowed the polyester to undergo acid catalysed ester hydrolysis when free protons were available.
- Resource Type
- DOI
- Date Created
- 2011
- Former identifier
- 833001
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